In vitro systems are being used to characterize the mechanisms by which cyclic nucleotides alter cell function and to explore the mechanism of action of agents whose effect on cell function is mediated by cellular accumulation of cyclic nucleotides. In turkey erythrocytes B- adrenergic catecholamines increase cellular cyclic AMP which in turn stimulates membrane transport of sodium and potassium. In isolated lymphocytes from rat thymus, protaglandin E1 and cholera toxin increase cellular cyclic AMP which then stimulates membrane transport of several neutral acids. Several gastrointestinal hormones activate adenylate cyclase in plasma membranes prepared from isolated pancreatic acinar cells and also alter various cellular functions; however, at present, it is not clear which, if any, of these functional alterations are mediated by cyclic AMP.